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How Pregnancy Changes a Runner’s Body

New research suggests that the biomechanical changes that occur during pregnancy can linger long afterward and significantly affect a woman’s running stride.

There’s never been any doubt, of course, that pregnancy drastically changes a woman’s body. She gains weight, her center of gravity shifts and connective tissues become looser. These changes are desirable and necessary for birth.

But they also affect how a woman moves.

A study published in March in the journal BMC Pregnancy and Childbirth, for instance, documented how women’s walking strides grew wider and shorter as their pregnancies progressed. They began, in essence, to waddle.

But there has been little science available about how pregnancy affects running form, in part because relatively few women have run during and after their pregnancies. That situation is changing, though, thanks in part to strong evidence that running throughout pregnancy does not harm a woman’s fetus, together with well-publicized success stories like those of the marathoners Kara Goucher and Paula Radcliffe, who trained throughout their pregnancies and returned to competition soon after.

But while more pregnant women and new mothers run today, many also experience unaccustomed aches and pains and a niggling feeling that somehow their stride is different now.

Such complaints drive many new mothers to the doctor’s office. And they also sent some to the running clinic at the University of Wisconsin in Madison, whose director, Bryan Heiderscheit, a professor of orthopedics and rehabilitation, could find no studies examining running biomechanics in the pregnancy and postpartum periods.

So he and his colleagues set out to conduct their own. So far, only a handful of women have been studied in depth, but the early results, presented at a 2013 sports medicine scientific meeting and published in The Journal of Orthopaedic & Sports Physical Therapy, are among the first to track biomechanical changes in a pregnant runner’s body and how those changes might effectively be managed.

One of the cases involved a 27-year-old runner who, beginning in the sixth month of her pregnancy, began visiting the university’s exercise lab every four weeks. There, she ran on a treadmill while the scientists used 3-D motion-capture technology to study her form. Her final visit while pregnant came two days before she went into labor. Six months later, she returned to the lab for a follow-up run. By then, she was running on her own about 15 miles a week.

The scientists found that as her pregnancy progressed, her pelvis began tilting forward more and more, altering to some degree how she landed with each stride. Six months postpartum, much of that forward tilt remained. She also displayed more side-to-side pelvic motion while running than she had when she began the study six months into her pregnancy.

She didn’t report any appreciable pain while running. But that wasn’t the case for another postpartum runner studied by Dr. Heiderscheit’s group — a 33-year-old mother of two who visited the running clinic 14 months after her second child was born because of considerable hip and low back pain while running.

After she ran on the lab treadmill, the researchers determined that her pelvis tilted abnormally far forward during running and moved too much from side to side. In effect, her pelvis remained unstable a year after her last pregnancy. Partly in consequence, her right leg struck the ground harder than the left, absorbing about 30 percent more force with each stride.

“None of this was surprising,” Dr. Heiderscheit says. “Pregnancy and labor stretch the muscles and connective tissues in the abdomen,” which allows the slightly unmoored pelvis to tilt and sway. Unless a woman strengthens the affected muscles after pregnancy, the tissues remain stretched.

Relying on that gym standby, crunches, won’t do the trick, though, Dr. Heiderscheit says. They don’t activate the small muscles deep within the abdomen. Instead, he suggests pulling the belly up and in multiple times and also “imagining that you’re trying to cut off the flow of urine.”

Using those techniques, together with traditional abdominal exercises like squats, planks and bridges (instructions for these can be found on The Journal of Orthopaedic & Sports Physical Therapy’s Web site), can help stabilize the pelvic area. They also suggested that the woman shorten her stride by about 10 percent to reduce the pounding that she experienced as she ran.

After seven weeks, the woman reported that her feelings of pain while running had dropped from a constant 9, on a scale of 1 to 10, to an occasional 3. She returned to running about 20 miles a week.

“I think it’s time we acknowledge that having a baby is going to change how you run,” Dr. Heiderscheit says.

The changes are not necessarily deleterious, he adds. There is no evidence that a majority of mothers are slower runners or more injury-prone after giving birth. “But things are likely to feel different,” he says, “and probably for a long time.”

Ongoing studies at his clinic are looking into whether some of the alterations in running form are, in fact, permanent.

For now, he says, his advice to pregnant or postpartum runners is to pay attention to signals from your body. If you feel pain while running, especially in the back or hips, slow down and consult a sports medicine specialist. After medical clearance, consider starting an exercise program designed to strengthen the abdominal muscles.

And if you need inspiration, the mother with the sore back and hips, Dr. Heiderscheit says, recently completed her first half-marathon.